I've been using Rails for a few years, but I never dug deeply into Ruby. This book is one of the best technical books I have read, it keeps you engaged, which is very touch to do with such a dry subject matter. I read cover to cover (except the reference in the back) and I feel very confident in my Ruby skills. I learned something new in every chapter, and have a more grounded understanding of Ruby. I have already used this book as a reference, and I expect to do the same for quite a while.

If you are using Ruby in any way, this book should be in your library.